Matias URDAMPILLETA - TEL

tel-00770488, version 1 - 7 Jan 2013
ARTICLE
B
Figure 1. (a) Scheme of TbPc2 molecule constituted by a
single magnetic Tb 3 ion coordinated by two phthalocyanine
ligands. One of the ligands is subtituted by a pyrene
group and six hexyl groups. (b) Raman spectra of the pristine
graphene (bottom, red), TbPc2 powder (middle, black),
and the hybrid system graphene and TbPc2 molecules (top,
blue). The G and 2D modes of graphene and the M band of
TbPc2 molecules are indicated.
graphene and the structural integrity of the molecule
after the grafting. With respect to the techniques traditionally
employed to study molecules on surfaces (AFM,
STM), Raman spectroscopy can probe structural and
electronic properties of both molecules and graphene
Figure 2. (a,b) Spatially resolved Raman intensity map of the TbPc2 M band and of the
graphene G band, respectively. Color scale range: 0 to 200 CCD counts from black to yellow.
(c,d) AFM topography image and height profile of the pristine graphene of the
grapheneTbPc2 hybrid system, respectively. Color scale range: 0 to 10 nm from black
to white. For panels a, b, and d, the sample was prepared with a solution concentration
of 10 8 mol·L 1 (scale bars 3 m).
in a fast and nondestructive way. 24,25 The enhanced Raman
intensity signal of these SMMs on graphene allowed
studies down to few isolated molecules on the
surface. The weak orbital overlapping between
graphene and SMM suggested by our experiments is
corroborated by ab initio calculations and electron
transport measurements.
RESULTS AND DISCUSSION
Figure 1b presents typical Raman spectra of pristine
graphene, TbPc2 powder, and the grapheneTbPc2
hybrid. Note that Raman scattering is resonant when
the excitation energy matches an electronic transition
of TbPc2, 15 which is the case at the excitation wavelength
used here. The Raman response of the
grapheneTbPc2 hybrid is a simple superposition of
the response of each component without any shift or
disappearance of a mode, indicating that both remain
chemically unchanged after the grafting process. The
spectrum obtained on TbPc2 powders shows several
peaks between 1050 and1650 cm 1 . In particular, the
M mode in Figure 1b is a doublet with 1512 and 1520
cm 1 , which was ascribed to the pyrrole CAC and aza-
CAN stretching modes, respectively. It was found that
these frequencies depend on the ionic radius of the
rare-earth complexes. 26,27 Since all Raman modes of the
TbPc2 molecule followed the same behavior in our experiments,
the most intense band (M) was used for the
Raman maps. As presented in Figure 2a, the spatially resolved
Raman map of TbPc2 molecules precisely
matches the graphene G band map (Figure 2b), while
no TbPc2 Raman signal is detectable on the silicon
substrate.
AFM measurements confirm this result
because the roughness on the graphene
after grafting is much more pronounced
than before grafting, whereas the roughness
hardly changed on the silicon oxide
(Figure 2d). The 23 nm high roughness is
associated with the formation of molecular
clusters of few molecules (5) packed
together. For concentrations higher than
10 5 mol·L 1 , the AFM topography reveals
the presence of much large clusters (up to
1020 nm in size), which are uniformly distributed
without any evidence for grafting
selectivity. Indeed, molecular clusters also
appear on silicon oxide, as confirmed by a
weak TbPc2 Raman signal (see Supporting
Information). These observations establish
that the grafting mechanism is selective
and favors deposition on graphene, which
is important for hybrid device fabrication.
Moreover, all TbPc2 Raman modes are
present on the Raman spectrum of the hybrid
with the same frequency position and
width as for the TbPc2 powder (Figure 1b).
VOL. XXX ▪ NO. XX ▪ LOPES ET AL. www.acsnano.org